The present invention relates to the art of packaging using multi-layer films, and, in particular, to a new composite multi-layer film for providing an oxygen and water vapor barrier to multi-layer film packages.
Packaging technology has over the years required the development of many disciplines. Currently, packaging technologists integrate elements of engineering, chemistry, food science, metallurgy, and other technologies in order to provide the consumer fresh, healthy food product. In those cases where packages are prepared from multi-layer film, it is desirable to be able to provide a barrier which does not permit passage of air or water vapor.
In recent years, containers produced out of multiple-layer flexible film, such as bags and pouches, predominate the marketplace. In order to utilize continuous multiple-layer flexible film, the industry generally employs form/fill/seal packaging techniques. The type of product packaged dictates whether or not the technique will include horizontal form/fill/seal packaging (HFFS) or vertical form/fill/seal packaging (VFFS).
It is important for the packaging artisan to be able to select a multi-layer film having optimum barrier properties for storage of the food items and be confident of providing a high quality seal using high speed packaging apparatus. For example, it is known that stereoregular polypropylene, e.g., oriented polypropylene, is quite useful in the manufacture of packages from flexible films. Using oriented polypropylene as a core layer, additional layers in the way of coatings, co-extrusions, laminations, and combinations thereof are added to improve barrier properties of the film. In certain cases, films can be prepared which exclude moisture and oxygen, but permit the passage of light. In other cases, it is also important to prevent light from passing through the film barrier. Barrier properties can also be modified and/or enhanced by treatments such as heat and flame treatment, electrostatic discharge, chemical treatments, halogen treatment, ultra-violet light, and combinations thereof
A primary concern for designing multiple-layer films for packaging is to ensure they can be processed on high speed form/fill/seal machinery. Form/fill/seal package apparatus operates by unwinding continuous film from bulk film rolls, followed by forming pouches therefrom, filling the pouches, and, finally, sealing the pouch closed. Thus, the film must have sufficient flexibility to undergo machine folding from a flat orientation to a folded condition, and be subjected to a sealing function which is part of high-speed packaging apparatus. In selecting the optimum multi-layer film for its barrier properties, high-speed unrolling and folding are a primary concern. An additional, and very important aspect of the packaging process, however, is the ability to effectively seal the pouch after it is filled with the product.
High-speed horizontal and vertical form/fill/seal apparatus include sealing functions at various stages of the packaging process. In a horizontal form/fill/seal apparatus, individual pouches are formed by folding the multi-layer film in half followed by providing vertical seals along the length of the folded web and separating the pouches along the seals formed by vertical sealing. (Optionally, the bottoms of the pouches can also be sealed). After the pouch thusly formed is filled, the top of the pouch is sealed.
Similarly, in vertical form/fill/seal apparatus, the continuous web is formed around a tube and the web is immediately joined together by a longitudinal sealing jaw as either a lap seal or a fin seal. A lap seal is depicted schematically in FIGS. 1 and 1a of U.S. Pat. No. 5,888,648 to Donovan, et al. A fin seal is depicted schematically in FIGS. 2 and 2a of U.S. Pat. No. 5,888,648.
A second sealing function is present in a VFFS configuration which consists of a combination top- and bottom-sealing section (with a bag cut-off device in between). The top-sealing portion seals the bottom of an empty bag suspended from the bag forming tube while the bottom portion seals the top of a filled bag.
As a consequence of processing high-barrier property multi-layer films in high speed form/fill/seal apparatus, damage can occur metallized layers for providing a barrier to oxygen and water vapor.
In order, therefore, to provide high-barrier multi-layer film with hermetic seals, several factors must be considered. It is important to provide a sealing capability at as low a temperature as possible in order to retain, among other things, stereoregularity imposed during orientation, little or no film shrinkage, retention of film and/or chemical additive properties, and highly consistent quality sealing capabilities. Furthermore, the film must have surface characteristics which permit it to be readily used on high-speed machinery. For example, the coefficient of friction must be such that it can be readily unrolled from a high volume roll of film and passed through the packaging machinery. Undesirable sticking or friction characteristics can cause bag imperfections and interruption of high-speed processing. Moreover, seals formed during process must have good seal strength.
More recently, the packaging artisan has been concerned with the ability to provide quality seals which preserve the freshness of the contents while providing the consumer with an easily openable and reclosable container. Innovations to date have been primarily concerned with the components of the seal material. For example, U.S. Pat. No. 3,202,528 to James describes a oriented polypropylene film having an adherent heat-sealable coating which includes a material from the group consisting of copolymers of vinylidene chloride and acrylonitrile, copolymers of vinyl chloride and vinyl acetate, chlorinated rubbers, nitrocellulose and polyamide which melts below 160xc2x0 C. and an acidic material provided in an amount of about 20 to about 60% by weight of the film forming material. This adhesive is coated and dried on the film. U.S. Pat. No. 4,020,228 to Eastes describes a gel composition which provides a heat sealable surface to polyolefinic materials or cellulosic sheet materials. U.S. Pat. No. 4,121,956 discloses an ionomer adhesive adhered to an outer ionomeric surface of package wrapping for attachment of labels.
U.S. Pat. No. 4,292,882 discloses an oriented heat-sealable anti-static polypropylene film manufactured by applying to a surface of a base polypropylene film a heat-sealable olefinic polymer containing between 0.2 and 10% by weight of an anionic hydrocarbyl sulfonate. Andrews, et al. also provide that a slip agent can be incorporated for ease of handling.
U.S. Pat. No. 4,389,450 to Schaefer, et al. describes a multi-layer packaging film in which the outer polymeric layers cooperate to provide a relatively constant coefficient of friction differential. This enhances the ability to use the film in high speed processing to form fin seal and lap seals. Schaefer, et al. have addressed the problem of providing the proper coefficient of friction for use of the film in high-speed processing apparatus.
U.S. Pat. No. 5,049,436 to Morgan, et al. discloses a multi-layer film which is hermetically heat sealable over a broad temperature range. The Morgan, et al. patent describes a heat-sealable layer which includes an ethylene-propylene copolymer and/or an ethylene-propylene-butene terpolymer with an inorganic anti-block agent and a fatty acid amide.
U.S. Pat. No. 5,153,074 to Migliorini discloses a high barrier film which has been metallized. The Migliorini ""074 patent describes a metallized multi-layer film having a polymer substrate at least one surface of which includes a maleic anhydride modified polypropylene homopolymer or copolymer, and at least one surface having a skin layer thereon of ethylene-vinyl alcohol copolymer, such skin layer having an aluminum layer directly thereon. The ethylene-vinyl alcohol copolymer layer provides excellent oxygen barrier properties and is described as ranging in thickness from 3 to 30 gauge units.
U.S. Pat. No. 5,888,648 to Donovan, et al. discloses a multi-layer film which is hermetically heat sealable. The thickness and composition of layers are selected to avoid tunneling in seals, as well as to avoid z-direction tears when sealed bags are opened.
While the references set forth above as well as other references and disclosures in the art of film packaging disclose attempts to overcome specific problems identified in high-speed packaging using multi-layer films, none of the references provide a technological solution which simultaneously provides rapid formation of an excellent hermetic seal in form/fill/seal packaging apparatus while avoiding damage to the barrier layer during the packaging process.
Therefore, the present invention is intended to provide an improved multi-layer film structure and a method for improving multi-layer film structures to overcome the problems set forth above and other problems associated with form/fill/seal processing.
There is provided a metallized biaxially oriented film comprising:
(a) at least one polypropylene layer; and
(b) a metallized skin layer comprising
(i) a blend of syndiotactic polypropylene and butylene-propylene copolymer; or
(ii) a blend of syndiotactic polypropylene and a graft polymer of a propylene polymer and maleic anhydride.